Spark plug

Abstract

A spark plug has an insulator, a center electrode disposed in an axial hole, a resistor disposed in the axial hole and a seal member disposed between the resistor and the center electrode in the axial hole. The insulator includes an inner-diameter decreasing portion and a small inner-diameter portion. The center electrode includes a head portion supported on the inner-diameter decreasing portion of the insulator. The spark plug satisfies the following conditions: 1.8 mm≤L; and Cp≤11 mm where, assuming a region of the insulator from a boundary of the inner-diameter decreasing portion and the small inner-diameter portion to a rear end of the seal member as a specific region, L is a length of the specific region; D1 is an average inner diameter of the axial hole within the specific region; D2 is an average outer diameter of the specific region; and Cp is L/log(D2/D1).

Having described the invention, the following is claimed: 
     
       1. A spark plug comprising:
 an insulator having an axial hole formed therein in a direction of an axis of the spark plug; 
 a center electrode disposed in the axial hole, with a front end portion of the center electrode protruding from a front end of the insulator; 
 a resistor disposed in the axial hole at a position closer to a rear end of the spark plug than the center electrode; and 
 a seal member disposed in the axial hole at a position between the resistor and the center electrode so as to connect the resistor and the center electrode to each other, 
 wherein the insulator includes: an inner-diameter decreasing portion having an inner diameter decreasing toward a front end of the spark plug; and a small inner-diameter portion located closer to the front end of the spark plug than the inner-diameter decreasing portion; 
 wherein the center electrode includes a head portion located at a position closer to the rear end of the spark plug than the small inner-diameter portion of the insulator and supported on the inner-diameter decreasing portion of the insulator; and 
 wherein the spark plug satisfies the following conditions: 1.8 mm≤L; and Cp≤11 mm where, assuming a region of the insulator extending from a boundary of the inner-diameter decreasing portion and the small inner-diameter portion to a rear end of the seal member in the direction of the axis as a specific region, L is a length of the specific region in the direction of the axis; D1 is an average inner diameter of the axial hole within the specific region; D2 is an average outer diameter of the specific region; and Cp is a value given by L/log(D2/D1). 
 
     
     
       2. The spark plug according to  claim 1 , wherein the spark plug satisfies the following condition: 2.0≤MIS≤3.0 where S is a maximum cross-sectional area of the axial hole within the specific region as taken perpendicular to the axis; and M is an area of contact between the seal member and the center electrode. 
     
     
       3. The spark plug according to  claim 1  or  2 , wherein the spark plug satisfies the following condition: D1≤3 mm.